Of these four World Heritage–listed caves 30km southwest of Santander, two – El Castillo and Las Monedas – are open for 45-minute guided visits (in Spanish). Booking ahead online is highly recommended, especially for the more spectacular El Castillo. As you explore 300m into the cave, you’ll see art almost as breathtaking as that of Cantabria’s famous Cueva de Altamira – and unlike at Altamira, this is the genuine article, not a replica.
El Castillo's 275 paintings and engravings of deer, bison, horses, goats, aurochs, handprints, mysterious symbols and a mammoth (very rare) date from around 39,000 to 11,000 BCE. One red symbol, believed to be 40,800 years old, is Europe's oldest known cave art. El Castillo also has exquisite cathedral-like rock formations. Las Monedas contains less art (black animal outlines, from around 10,000 BCE), but has an astounding labyrinth of shimmering stalactites and stalagmites.
Five to seven daily buses run from Santander to Puente Viesgo (€2.45, 40 minutes), from where it's a 2km uphill walk or taxi ride to the cave.